Men concerned about contraception may soon be able to use the male equivalent of the Pill, without the potential side-effects of a drug based on altering the balance of sex hormones.

Scientists have developed a chemical contraceptive that temporarily blocks the development of sperm but does not interfere with testosterone levels.

Trials on laboratory animals have shown that the contraceptive effect is reversible and that there are no apparent long-term side-effects. Human trials of the new male contraceptive could begin within the next few years.

A male contraceptive pill has proved difficult because of a basic biological fact of human reproduction: a man naturally produces about 150 million sperm each day of his adult life.

Existing chemical contraceptives for men under development are based on altering the levels of sex hormones circulating in the bloodstream in order to inhibit the natural state of male hyper-fertility.

A male contraceptive based on hormones is undergoing final clinical trials. It is made of progesterone - a female hormone that inhibits the formation of sperm - and the male hormone testosterone, added to counter the adverse effects of progesterone.

However, many men fear that a male pill based on introducing female hormones into the body may affect their virility or other aspects of masculinity, even though the results of the first clinical trials have found no evidence for this.

An alternative approach being pioneered by Chuen Yan Cheng of the Population Council in New York relies on a chemical called adherin which interferes directly with the way key cells in the testes help to nurture the development of mature sperm. Immature sperm cells in the testes develop with the help of "nurse" tissue called Sertoli cells, but adherin breaks the close bond between Sertoli cells and the cells that would otherwise develop into sperm.

"It only works at the site of attachment with the Sertoli cells so they don't affect the hormones of the male. This is a new and different approach to the hormone-based contraceptives," Dr Cheng said.

"We're not saying that the hormonal approach is no good - it's one of the best standard methods in male contraception - but it's important to give the consumer a choice," he said.

Results of a trial on laboratory rats, published in the journal Nature Medicine, showed that sperm production fell to levels that in humans would render a man infertile.

Adherin on its own is known to have toxic effects on other parts of the body but Dr Cheng and his colleagues overcame the problem by binding it to a synthetic version of follicle-stimulating hormone which, in men, attaches itself to only one cell in the body - Sertoli cells.

"For a male contraceptive even a very low level of toxicity is unacceptable which is why we adopted the approach of using follicle-stimulating hormone," Dr Cheng said.

Tests on vital organs, such as kidneys, liver, brain, heart, prostate gland and other cells of the testes showed that the drug had no effect, he said. In men the drug would have to be taken as an implant rather than an oral pill as it is broken down in the digestive tract.

The rats quickly regained their ability to produce normal levels of sperm once the drug had been cleared from their bloodstream. In humans this would be a critical feature if the drug was to be acceptable to the public, Dr Cheng said.

"If you give the consumer piece of mind that their fertility will be restored, and that their hormones are not going to be affected, they may feel that it is safer to use this contraceptive," he said.